1. Field of the Invention
This invention relates to a system for controlling an automatic vehicle transmission, more particularly to a system for, and a method of, controlling an automatic vehicle transmission.
2. Description of the Related Art
Prior-art automatic vehicle transmissions are provided with multiple ranges (positions), typically P, R, N, D4, D3, 2 and 1. In the automatic gearshift range D4, gearshift is conducted automatically in accordance with a gearshift scheduling map (characteristics) stored in an on-board microcomputer memory, based on the vehicle speed and the throttle opening (indicative of the engine load) as parameters indicating the vehicle operating conditions. The vehicle driver ordinarily drives with the shift lever setting in the D4 range. This prior-art automatic gearshift mode is hereinafter referred to as "auto-shift mode" or "auto-shifting means" in the specification.
Since in the D4 range in the auto-shift mode, gearshift is conducted independently of any desire on the part of the driver, if the driver desires to downshift for sporty driving or for utilizing the engine braking effect, he must manually move the shift lever from the D4 range to the D3 or 2 range. On the other hand, when he again desires to upshift from the gear, he must return the shift lever to the D4 range. This means that the driver is required to frequently move the shift lever when running on a winding or hilly road. Although in the D3, for example, it is possible to run at 1st, 2nd or 3rd gear, hence it is sometimes difficult for the driver, contrary to what has been expected, to obtain the desired gear.
Therefore, as taught by Japanese Laid-Open Patent application Nos. Sho 59(1984)-37359, Hei 2(1990)-8545 and Hei 6(1994)-74318, it has been proposed to provide, in the automatic vehicle transmission, a switch or lever separate of the shift lever which enables the driver to upshift or downshift by manual operation of the switch or lever. The use of such a device will hereinafter be referred to as "manual-shift mode" or "manual-shifting means" in the specification.
When the driver keeps a certain gear in the manual-shift mode, the acceleration might sometimes be poor due to the engine speed drop, or engine braking effect might become insufficient at times if the gear is higher relative to the vehicle speed, degrading drivability or driver's comfort. In order to solve these problems, therefore, it has been proposed to provide an automatic downshifting function in the manual-shift mode. The automatic downshifting function operates to downshift when the vehicle traveling speed has drooped below a reference or threshold speed. The reference speed is predetermined differently for the respective gears. The operation of the automatic downshifting function will hereinafter referred to as "sub-auto-shift mode" or "auto-downshifting means".
However, when the vehicle is climbing a hilly road with the manual shift mode and the sub-auto-shift mode in operation, the drivability or driver's comfort becomes worse due to the running resistance increase until the vehicle speed has dropped below the reference speed. Similarly when the vehicle climbs a hilly road, the driver may sometimes feel vehicles acceleration speed great. The downshifting is not effected, however, until the vehicle speed has dropped below the reference speed, also degrading the drivability or driver's comfort.